1. Field of the Invention
The present invention relates to the field of integrated circuit fabrication, specifically to the field of microlithography.
2. Description of the Related Art
Modern integrated circuits are getting more dense and compact. To accommodate smaller features, new photoresists are being used. Currently, UV wavelengths of 248 nm and 193 nm are being used for microlithography tools. Soon, the exposure will be carried out at even lower wavelengths, including 157 nm. These low wavelengths will allow for the use of sub-70 nm features. To remove photoresist, a chemical distinction is provided between exposed and unexposed resist. This is primarily done through the use of photo acid generators (PAG), photoactive materials that form an acid upon UV exposure at the proper wavelength. That acid buildup allows for selective removal of exposed photoresist, while leaving unexposed resist in place.
A problem with small photoresist wavelengths arises from interfacial interactions between the resist and the underlying layers. One such problem is that the acid generated upon exposure diffuses into underlying layers. Because some of the acid has diffused out of the resist, sufficient acid may not remain to facilitate the removal of all of the exposed resist. Some of the exposed resist is then not removed from the surface of the underlying layer, leaving a “resist scum” that can cause fabrication failures. The resist scum can cause footing along the edges of the exposed photoresist, decreasing the width of the openings in the pattern. Ultimately, the scumming or footing of the resist on the surface of the underlayers could change the critical dimensions (CD) of the device. CD is defined as the dimensions of the smallest features (e.g., width of interconnect line, contacts, trenches, etc.) defined by photolithography in the course of semiconductor device/circuit manufacturing using a given technology.
Recently bottom anti-reflective coating (BARC) has been used beneath low wavelength photoresist. However, in some circumstances, the use of a BARC is impracticable due to integration issues. For example, when the BARC layer is organic, the etchant that removes the BARC can possibly damage any underlying organic layer.
Some methods have been tried to combat the diffusion problem with short wavelength photoresist. One such method is to put a thermal acid generator (TAG) into a BARC material. One example of this is U.S. Pat. No. 6,329,117, issued to Padmanaban, et al. When the TAG is heated, it creates an acid in the BARC. This higher acidity level discourages acid diffusion. However, this method usually requires a post-exposure bake (PEB) or other heating process. Additionally, in some fabrication processes, a BARC layer is not desired between the photoresist layer and an underlayer.
U.S. Pat. No. 6,528,235 issued to Thackeray et al. on Mar. 4, 2003 describes an anti-reflective coating layer, called an anti-halation layer, with an acid additive, such as acid generators and acids. The layer can be removed during resist stripping and easily etched through when etching the underlying substrate. The optional acid additive is added to catalyze the crosslinking reaction between a resin binder and the crosslinker. The anti-halation layer bonds to the photoresist during a thermal process.
Each of the above methods faces difficulties and disadvantages. Therefore, there is a need for methods and procedures for the prevention of resist footing and scumming.